Quantum computing machine under scrutiny

Mar 18, 2014

A new and innovative computing machine is currently attracting a great deal of attention in specialist circles. A team under the leadership of Matthias Troyer, a professor at ETH Zurich, has now confirmed that the machine uses quantum effects. However, it is not any faster than a traditional computer.

D-Wave – a special computing machine with this name has been getting computer scientists and physicists talking for a number of years now. The Canadian technology company of the same name is advertising the machine as a quantum computer. However, whether or not the machine does in fact use quantum effects is the subject of controversial debate amongst experts in the field. If it does, then this would make D-Wave the world's first commercially available quantum computer.

The company sold its system to illustrious customers, piquing the interest of the scientific community and of bloggers and journalists even further. For example, the very first machine was sold to the US arms manufacturer Lockheed Martin in 2011, which provided it to the University of Southern California in Los Angeles for tests. Last year, Google purchased the second machine. D-Wave can solve certain mathematical problems referred to as optimization problems by searching for and finding the state of lowest energy in a system. That is why the technology is of interest to this company.

Analogue device, not a quantum computer

But the question of whether or not D-Wave does in fact use quantum effects is not the only disputed aspect of the machine. Scientists and bloggers have also expressed doubt as to whether the machine can be accurately described as a computer at all. There are also different opinions regarding whether or not it can compute faster than a traditional computer. To find answers to these questions, Matthias Troyer, a professor at the Institute for Theoretical Physics at ETH Zurich, worked together with colleagues at the University of Southern California in Los Angeles and tested the system located there.

In their study, which has now been published in the journal Nature Physics, the Swiss-American team of researchers comes to a conclusion that is not clear cut. On the one hand, the scientists confirm that D-Wave does in fact use quantum effects. However, in other areas the researchers are more critical: "D-Wave is an analogue device, a prototype that can be used to solve optimization problems. It would be more accurate to describe it as a programmable quantum simulation experiment", says Professor Troyer, an internationally recognized expert in the field. "D-Wave is certainly not a universal quantum computer."

Quantum effects, but only momentarily

The researchers came to their conclusions by writing thousands of computing problems of differing complexity and solving each of these one thousand times on three systems: once on D-Wave and twice on a simulation programme for optimization problems that ran on a traditional computer. The simulation programme ran in two modes, where one took quantum effects into consideration and one did not. For each task, the scientists made a note of how often which system delivered the right solution. It turned out that D-Wave behaves in the same manner as the simulation that accounted for quantum effects but differently from the simulation that did not.

The scientists were amazed by this result, because the quantum effects of D-Wave are extremely short-lived, lasting only a few billionths of a second. Physicists describe this as coherence time. Because it generally takes around 500 times longer to solve an optimization problem, most experts assumed that the quantum effects with D-Wave simply could not play any role. And yet they do, as the results of the researchers have shown. "It appears that the quantum effects do not necessarily have to be coherent all of the time in order to have a significance", explains Troyer.

Not faster than a traditional computer

When one considers that research into quantum computers is carried out primarily because of the promise of hugely accelerated computing speeds, then another conclusion arrived at by the researchers is particularly significant, namely that D-Wave is not faster than a traditional computer.

The speed of D-Wave is the subject of intense debate amongst experts in the field, particularly since a publication by a computer scientist at Amherst College caused uproar in May of last year. According to the publication, depending on the computing problem, D-Wave is several thousands of times faster than a traditional computer. The researcher examined a version of D-Wave that almost corresponds to the current version, in existence for one year, with a computing capacity of 512 quantum bits (qubits). By contrast, the study carried out by the researchers from ETH Zurich is based on a predecessor version with 108 qubits.

"Not only have we demonstrated that a traditional computer is faster than the 108-bit version of D-Wave", Troyer responds. "We also used a traditional computer to solve the same problems that can be solved by the new 512-qubit version or hypothetically even higher-performing machines." When these findings are compared with those of the researcher from Amherst College, it becomes clear that D-Wave is consistently slower than a traditional computer for the tests performed. According to Troyer, the problem with the Amherst study is that it compared fast algorithms for D-Wave with slower algorithms for traditional computers. "We developed optimized algorithms for traditional computers. This allows us to match even the current 512-qubit version of D-Wave", explains Troyer. "Nobody knows at present whether a future quantum system like D-Wave with more qubits will offer any advantages over traditional systems. This is an important question, and we are currently using experiments on the 512-qubit machine to find the answer."

Quantum annealing with D-Wave

An imperfect crystal structure made of metals or glass can be improved by heating the material until it glows and then cooling it in a controlled environment. In the hot material, the atoms have a certain freedom of movement and can realign in a more refined crystal lattice. This craft technique is thousands of years old and called annealing. A comparable method has also been in use for the past 30 years in computer science as an optimization process and is called annealing as well.

A typical question that can be answered using this method is the search for the lowest point of a landscape. To understand this better, it is possible to imagine a thought experiment where a sphere located in a landscape is subjected to jolts depending on temperature. At high temperatures, the sphere can hop around the entire landscape. The lower the temperature, the harder it is for the sphere to cross mountains. If an experiment is repeated several times, starting with high temperatures and slowly cooling, at the end of the experiments the sphere will frequently be found at the lowest point of the landscape.

When the D-Wave system solves an optimization problem, it uses a similar procedure. In addition, quantum physics and thus tunnel effects also have a role to play: the sphere (remaining with the above example) is also in a position to tunnel underneath the mountains in the landscape. With D-Wave, however, it is not spheres that are moving. Instead, individual superconducting circuits act as quantum simulations or artificial atoms. For this purpose, the system must be cooled to temperatures of almost absolute zero. The circuits simulate the spin of atoms. There is the spin "up" and the spin "down" as well as (because quantum physics plays a role) superposition of the spins, the state of "both up and down". In the D-Wave circuits, the spins are simulated by the direction in which the electrical current is flowing. Physicists call the optimization procedure used by D-Wave "quantum annealing".

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Analogue is not quantum, it's analogue. It's not digital, but it's even more not quantum.

I've been saying it's an analogue computer since 2007... yes tunneling is a quantum effect, but it's not doing anything an analogue or digital system can't - here, the quantum effects are incidental, and trivial.

It's a "quantum device" in the same sense as a cathode-ray telly, or CD player....

It's intriguing, by following the publishing cycle, the mainstream science portals are far behind the blogs on this story. Since the pre-print paper has been out for quite a while this has been discussed on science blogs for several months now: http://wavewatchi...riments/

Point is, a true quantum computer could do things an analogue or digital can't...

Like what?

Well IMHO a true quantum computer would be using quantum strangeness to perform functions classical machines can't, such as factorising a 50 digit number in the same time it took to do a 2 digit number - near-instantaneously - rather than sifting through a sequential 'brute force' approach. When that day comes, the revolution will have arrived...

true quantum computers can calculate designed optimization problems in essentially 1 flop. traveling salesman is a good example. Classical computers have to add a whole bunch of stuff and try all the possible solutions. Presuming you have a quantum computer with sufficient q-bits, Upon feeding, the optimization patern 'falls out' of the system as the path of least resistance.

due to the speed of decoherence, the real trick of quantum computers won't be in having q-bits (those have been around for ages), but being able to feed and read them faster than the state degrades. it'll play a role in anything that's currently being heavily brute forced. hashing.. large factorials, large factors. a few kilo-q-bits will be enough for anything conventional computers do.

This first step as "programmable quantum simulation experiment" is the right one to begin a simple quantum computer, starting with the quantum reality in which we live, and because quantum real effects cannot be simulated with classical computer for more than 10 particles, this type of quantum simulation will be quite faster in the future. For example the high temperature superconductivity cannot be simulated with classical computers, starting from a quantum model and calculate the critical temperature for this model. A quantum simulator will be able to make this simulation, very useful even with decoherence like in the reality.Thus I am sure that "programmable quantum simulation experiment" will be extremely useful in the future, because programmable classical simulation experiments are not able to simulate "quantum simulation experiments" in a human life time, for real complex systems.It is a beginning that must be worked all out and continued.

IamVal's answer was good, but I'd like to suggest that it's not so much that regular computers "can't" do it. It's more a matter of practical time frames I think. From what I can see, the most powerful approaches in computing involving a quantum computer will be a hybrid system that uses a quantum module kinda like a graphics card, to solve specific types of tasks for the main processor. An example of this might be a database server that uses a quantum chip to do database query and indexing routines really fast. This might be important to someone like google, because they might replace 5 story buildings full of servers with a single machine. Well, maybe.

The knock-on implication however is that a sufficiently-powerful quantum co-processor could crack any form of encryption instantly, rendering most current forms of cyber security obsolete at a stroke, hence once such a technology becomes viable, everyone will want to switch over to it ASAP, if only for security, over performance.

However, much of QC's parallelism is shared by analogue devices, too; thus the question arises whether a machine like D-wave is really exploiting quantum effects to a meaningful advantage... for instance a cathode-ray tube is using quantum principles to the same effect as a digital LCD panel, but if i sold old CRTs as revolutionary new "quantum VDUs" i'd be somwhat overstating their novelty...

Like anyone else i look forward to the benefits of the QC revolution, but until it can crack RSA etc. we've a ways to go IMHO..

The knock-on implication however is that a sufficiently-powerful quantum co-processor could crack any form of encryption instantly, rendering most current forms of cyber security obsolete at a stroke, hence once such a technology becomes viable, everyone will want to switch over to it ASAP, if only for security, over performance.

However, much of QC's parallelism is shared by analogue devices, too; thus the question arises whether a machine like D-wave is really exploiting quantum effects to a meaningful advantage... for instance a cathode-ray tube is using quantum principles to the same effect as a digital LCD panel, but if i sold old CRTs as revolutionary new "quantum VDUs" i'd be somwhat overstating their novelty...

Like anyone else i look forward to the benefits of the QC revolution, but until it can crack RSA etc. we've a ways to go IMHO..

Jizby

I forget what the letters stand for (the inventor's names i think) but it's a standard 'hard' encryption algorithm, whose security is based on the computational difficulty of factoring large primes. I only mention it as an example - there's plenty of others built on similar principles, used in all sorts of industries from communications to finance and commerce etc.

Once these are amenable to QC decryption, everyone using them will be forced to upgrade to quantum encryption methods if they want to remain secure - it'll be day zero...

Suffice to say, in light of recent revelations on certain agencies activities the momentum behind this revolution will be considerable; it'll basically snowball almost overnight. Everyone will need it, those possessing it will have guaranteed confidentiality, and everyone else will be sitting ducks...

When this storm hits it's gonna be a big deal. For now, D-wave is just rumble over the horizon.

You're conflating thermodynamic efficiency with functional efficacy. Sure quantum computers will generate heat, at least until they can be built using passive superconductive parts. And they'll produce errors... but then so do CD players and flash memory sticks. That doesn't mean data integrity can't be guaranteed via error-correction, redundancy etc.

It's not the practicability of QC i'm skeptical about, just the imminent forecast. But for now it looks set to remain mostly sunny, even if the threat remains of somewhat more unsettled conditions further ahead...

Quantum tunneling (per D-wave) is instantaneous - there's no delay or journey time, and this is fundamentally different to how your processor is intended to work (ie. within its specified operating temperature range). Definitely a cooling problem IMHO...

Jizby

Tunneling in classical processors is due to excess heat - a function of current and resistance, hence as fabrication density increases it becomes a hard limit. But this is an incidental consequence of excess heat, not an intrinsic limitation of digital processing per se. D-wave actually uses tunneling electrons as its qubits - or rather, the probability of a tunneling event. And of course, one of the early controversies following the discovery of single-electron tunneling is the absence of transport speed - the electron crosses the boundary instantaneously, effectively jumping positions without literally moving through space. It is precisely these sorts of spooky tricks that form the basis of quantum processing operations.